1. Field of the Invention
The present invention relates to a cooling system of a furnace, more particularly, to a multi-cycle cooling system, located by the furnace door. We can decrease the probability of pollution by particles and extend the lifetime of the furnace devices by using different liquid and gas cycles, cooling the different steps the semiconductor processes in the furnace.
2. Description of the Prior Art
Thermal processing furnaces are used to provide high temperature processing of semiconductor based wafers which are placed within a gas filled processing chamber. Several silicon, silicon based or other types of wafers are typically used in manufacturing semiconductor thermal processing. To avoid the contact of wafers with human hands, the wafers are mechanically inserted into the furnace. In vertical atmospheric diffusion furnaces the wafers are usually inserted into a tower which is on top of a pedestal assembly. The pedestal assembly vertically transports the tower into a tubular shaped chamber of the furnace where the thermal processing occurs. Using a similar principal, horizontal processing furnaces utilize a cantilever assembly which supports wafers therein. The cantilever assembly is transported along a horizontal axis into a horizontally oriented tubular shaped processing chamber. In either type of furnace, it is desired that the wafers be protected from exposure to undesirable gases to ensure proper thermal processing. For example, oxygen, water vapor and particulate found in air can react with the wafers and adversely effect their chemical and physical properties.
In order to ensure the purity of the reactions, the processing chamber of the furnace must be adequately sealed. Therefore, in a thermal processing furnace the interface between the pedestal assembly and processing chamber is designed to prevent reactant gases, from within the processing chamber, from leaking therethrough. Typically, in thermal diffusion furnaces the pedestal assembly is sealed from the tubular processing chamber by an O-ring. Moreover, the tubular processing chamber is usually mounted on a base and is also sealed thereto by an O-ring.
In the present semiconductor processes, the thermal process is generally used in many different kinds of processes like deposition process, oxidation process, and other kinds of processes. Wafers must be heated to a temperature before the thermal process. Therefore, the furnace will still be a tool to be used in the thermal process in the present semiconductor process.
Two types of furnaces are used: One is a horizontal type furnace, and the other is a vertical type furnace. The basic structures of these two types are the same. FIG. 1A shows a structure diagram of the horizontal type furnace. At first, the wafers 10 which need heat treatment are placed on the wafer boat 11, which will be latter transported to the chamber by the paddles 12. The chamber is inside the process tube 13 of the furnace. After the wafer boat 11 arrives at the correct place in the chamber, the door of the furnace 14 must be closed to prevent outside gases and tiny particles from entering into the chamber, not to pollute the wafers 10. The closed door can also prevent the gases leaking out of the chamber from endangering workers when a chemical reaction, such as vapor deposition is taking place in the chamber. In order to make sure the temperatures of the wafers are the same in the chamber, outside the process tube 13, there is a heater 18, which is used to adjust the temperature inside the chamber. This heater is often divided into several parts to adjust each temperature for different regions in the chamber, and also to increase the usability of the furnace.
FIG. 1B shows a cross section diagram of the furnace. The objective of the heat treatment in the furnace is to proceed with semiconductor processes like vapor deposition or thermal oxidation. The reactant is transported through the fluid tube to the chamber by the fluid injector. The pump 19 and the tube 195 extract the by-products that are produced in the reacting process and the remnant reactants after the reaction. The tube 195 connects the process tube 13 and the pump 19.
In order to make the contacting area between the door and the process tube airtight when the door 14 contacts the process tube 13, the furnace uses a flange 17 and an end O-ring 142 on the process tube 13 and near the door 14. The objective of the flange 17 is creating a airtight space after the door 14 closes on the process tube 13. The objective of the end O-ring 142 is to prevent the gas inside the process tube 13 from leaking out.
The flange 17 on the process tube 13 near the door 14 is used to make the process tube 13 connect hermetically with the door, and to make sure that the process tube is an airtight place. Therefore, the end O-ring 142 must be assembled in the flange 17 to prevent the gas escaping out of the chamber from affecting the process in the process tube 13. When the process temperature is higher, the materials of the end O-ring 142 lose their elasticity and will not have enough ability to seal up the crevice between the door 14 and the process tube 13. Therefore, the flange 17 on the process tube 13 near the door 14 includes a cooling system to protect the end O-ring 142. Especially in the high temperature processes inside the process tube 13, cooling systems having higher efficiencies to extend the lifetime of the end O-ring are needed.
FIG. 1C shows part of a cross section diagram of the furnace. Traditional flange 17 comprises the first flange 181, the second flange 182, the flange O-ring 183, the end O-ring 142, and the cooling system 184 which is located in the first flange 181 and the second flange 182. This cooling system is a single cycle system. The cooling liquid flows into the second flange 182 and flows out of the first flange 181 after the cycle. Obviously, the liquid cooling cycle is used for cooling and protecting the flange O-ring 183 and the end O-ring not to lose their capabilities in the high temperature process. In the traditional flange cooling system, the cooling liquid is usually used in water. The boiling point of the water is about 100xc2x0 C., therefore the temperature of the place near the flange will be limited in the cooling liquid which flows in the cooling system and the temperature of the place near the flange will not be raised at will. In other words, the temperature of the place near the flange is lower than others places on the process tube.
In the semiconductor process like the deposition process or the thermal oxidation process, the gas in the process tube usually proceed in the chemical reaction and produce some by-products in the reaction process. Following the property of the by-product is different, the critical temperature which divides by-products into the vapor phase and the solid phase is different. When the condition of the temperature is higher, the probability of the by-products becoming to the vapor phase is higher. When the conditions of the temperature is lower, the by-products become to the solid phase easier. The solid phase by-products deposit easily on the place around inside the process tube to affect the qualities of the wafer in the process.
In general, the pump 19 that is on the back-end of the process tube 13 is used to extract the by-products, which are produced in the semiconductor process, from the process tube 13, but the by-products cannot be extracted from the process tube 13 absolutely. Sometimes, the remainder by-products in the process tube 13 are in the vapor phase, but sometimes the remainder by-products are usually in the solid phase depositing at the lower temperature place around inside the process tube. Furthermore, because we use pump 19 to extract the by-products from the process tube 13, the solid phase by-products may be brought to the pump 19 or the tube 195 which connects the pump 19 and the process tube 13 by running pump 19 to cause the process problems. Obviously, it may be found that the vapor phase by-products are extracted by pump 19 continuously and not cause some serious problems. It may also be found that the solid phase by-products not only deposit on the wafers but also deposit on the pump 19 and the tube 195 which connects process tube 13 and the pump 19. This situation makes wafers 10 which complete the process to be scraps and decreases the efficiency in extracting the by-products from the process tube 13 by the pump 19. Therefore, the solid phase by-products are the pollution sources which are not ignored in the furnace proceeding semiconductor process.
For example, when a low pressure chemical vapor deposition process is performed to form silicon nitride, NH4 and SiCl2H2 (DCS) reciprocally interacts under 800xc2x0 C. and 150 mill torrs environment. Herein, the ratio of NH4 to SiCl2H2 is about 3:1 and attendant by-product is NH4Cl. Further, because gas-like NH4Cl is solidified when temperature is less than 150xc2x0 C., it is possible that white particles of NH4Cl are formed in chamber terminal. If the liquid cooling system which is using water, NH4Cl will become from vapor phase to solid phase white particles and deposit on the door 14 terminal in the process tube 14. Because the boiling point of the water is 100xc2x0 C., and this temperature is lower than the temperature in NH4Cl becoming from vapor phase to solid phase white particles. The deposited particles will pollute the wafers 10 which are in the process tube 13 and the tubes which connect to the process tube 13 by the pump 13 extracting continuously. The pump 13 is on the end terminal of 13.
In the foregoing discussions about furnace, it may be found that when the furnaceis used, a cooling system must be used in the flange to protect the end O-ring and the flange O-ring, therefore the solid phase particles deposit around the flange easily. The solid phase particles will pollute the wafers to become the scraps after proceeding the process or affect the proceeding of the pump, even decrease the lifetime of the pump. Therefore, the structure of the furnace must be changed to decrease the possibility of forming the solid phase particles. The present invention is especially in changing the cooling system of the flange.
With this as background, the solid phase by-products create a lot of problems in the furnace process in using traditional cooling system. The major objective of the invention is to decrease the probability in particle pollution in the furnace, especially to decrease the particle pollution which is came from the solid phase by-products in the process tube in the semiconductor process.
The second objective of this invention is to decrease the probability in the solid phase by-products depositing around the cooling system of flange to decrease the bad rate of the wafers after thermal process.
The third objective of this invention is to provide a cooling system in the flange. This cooling system can be used in different range of the temperature to reach the cooling function and can decrease the probability in forming the particles.
The forth objective of this invention is to decrease the temperature of the end O-ring and the flange O-ring in the proceeding process effectively by using the cooling system in the flange to extend the lifetime of the end O-ring and the flange O-ring.
The fifth objective of this invention is to decrease the probability in forming solid phase by-products by using the multi-cycle cooling system in the flange to decrease the cost of the production.
It is a further objective of this invention to decrease the probability in forming solid phase by-products by using the multi-cycle cooling system in the flange to increase the cycle time in the process. This situation also can decrease the times in cleaning furnace and increase the efficiency in the proceeding process.
According to the foregoing objectives, the present invention provides a multi-cycle cooling system to make the temperature of the place around the end O-ring and the flange O-ring higher than the critical temperature which divides by-product into the vapor phase and the solid phase to decrease the probability in forming solid phase by-products. This situation makes the wafers which are proceeding process not to be polluted by solid phase by-products depositing on the surface of the wafers to increase the probability in producing scraps. This multi-cycle cooling system can protect the end O-ring and the flange O-ring not to be damaged from the high temperature to extend the lifetime of the end O-ring and the flange O-ring. This multi-cycle cooling system also can increase the cycle time of the proceeding process and decrease the times in cleaning the furnace. This multi-cycle cooling system further can increase the efficiency of the process and decrease the cost of the production.